1. Field of the Invention
The present invention relates to a copier, printer, facsimile apparatus or similar image forming apparatus and more particularly to a developing device included in such an image forming apparatus and a developing roller for the developing device.
2. Description of the Background Art
It is a common practice with an electrophotographic image forming process to use a two-ingredient type developer, which is a toner and carrier mixture. The developer deposits on a developer carrier in the form of a magnet brush and contacts an image carrier on which a latent image is formed. A bias for development is applied to the developer carrier, forming an electric field between the image carrier and the developer carrier. As a result, the toner of the developer is selectively transferred from the developer carrier to the image carrier, developing the latent image.
The developer carrier is generally implemented as a sleeve accommodating a magnet roller therein. The magnet roller forms a magnetic field for causing the developer to rise in the form of a magnet brush on the sleeve. More specifically, the carrier of the developer rises on the sleeve along the magnetic lines of force issuing from the magnet roller, forming brush chains. The toner, which is charged beforehand, deposits on the brush chains to thereby form a magnet brush. The magnet roller has a plurality of magnetic poles implemented by a rod-like or similar magnet each. Among them, a main pole for development is positioned in a developing zone for causing the developer to rise on the sleeve.
At least one of the sleeve and magnet roller moves to convey the developer risen on the sleeve to the developing zone. In the developing zone, the developer rises along the magnetic lines of force issuing from the main pole, forming brush chains. The brush chains contact the image carrier while yielding. While the brush chains rub themselves against the latent image on the basis of a difference in linear velocity between the developer carrier and the image carrier, the toner is fed from the developer to the latent image. The developing zone refers to a range over which the magnet brush risen on the developer carrier contacts the latent image.
The problem with the conventional developing device using a magnet brush is that an image forming condition for increasing image density and an image forming condition for desirably reproducing a low-contrast image are not compatible with each other. It is therefore difficult to improve both of a high density portion and a low density portion at the same time. More specifically, a high image density is attainable if, e.g., a gap between the image carrier and the sleeve is reduced or if the developing zone is broadened in width. On the other hand, a low-contrast image can be desirably reproduced if, e.g., the above gap is increased or if the developing zone is narrowed. It follows that it is difficult to satisfy such contradictory conditions at the same time over the entire image density range.
For example, when importance is attached to the reproduction of a low-contrast image, the trailing edge of a black solid image or that of a halftone solid image is lost. This is also true with the crossing portions of solid lines. Other defects apt to occur are that a horizontal line originally identical in width with a vertical line appears thinner than the vertical line when developed, and that solitary dots are not developed at all.
Japanese Patent Laid-Open Publication No. 2000-29637, for example, discloses a method and an apparatus for development configured to implement high image quality over the entire density range by satisfying the contradictory conditions at a high level. However, a problem with this method and apparatus is that the main pole of a developing roller has a smaller angle between poles than the conventional developing roller, resulting in the need for magnets with high magnetic characteristics. Moreover, the main pole must be provided with accuracy of xc2x11xc2x0 that is higher than the conventional accuracy of xc2x12xc2x0. For high magnetic characteristics, the main pole of the developing roller is often implemented by Nexe2x80x94Fexe2x80x94B or similar so-called rare-earth magnets. However, rare-earth magnets are generally more expensive than ferrite magnets and therefore increase the cost of the developing roller.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 2000-81789 and 2000-323322.
It is an object of the present invention to provide a low-cost developing roller easy to produce and having high magnetic characteristics and an accurate main pole, a developing device insuring high image quality over the entire density range with the developing roller, and an image forming apparatus including the developing device.